The experiments presented in this work are aimed at gaining a greater understanding of the semantic impairment in semantic dementia (SD) in terms of modality-specificity and category-specificity, and to consider the implications for the organisation of the semantic system. To this end, the semantic abilities of a group of twelve SD patients with varying impairment severity were examined using a variety of methods - tests from a traditional semantic battery and novel tests, which examined the verbal and nonverbal knowledge of concepts and the semantic attributes of those concepts. The methods of assessment were directly contrasted and relationships between them explored. Throughout this work, comparisons with normal performance were drawn using data collected from age-matched healthy subjects. Longitudinal analyses of the performance of a subset of the patients were also presented to investigate any decline in patient performance over time. The findings suggest a remarkable degree of consistency in semantic performance in the patient group, regardless of stimulus modality or feature type, with few exceptions. However, this consistency was not reflected in the influence of semantic domain. Some patients showed category-specificity while others did not. These differences could not be explained by reference to psycholinguistic variables or evolutionarily determined categories. Differential processing of feature types was more satisfactory as an explanation but required the implication of more fine-grained distinctions than the binary sensory/functional knowledge classification. Recent models of which consider multiple principles of organisation within the semantic system are more likely to be able to account for all the data showing both consistency and inconsistency within the present cohort of SD patients, and the myriad findings in the semantic memory literature.